Signal transmitting cable

ABSTRACT

A fibre optic cable includes a core of primary coated optical fibres embedded in an inner layer of acrylate material, having sufficient tensile strength when cured to lock at least the outermost fibres in place and still allow the fibres to be easily broken out of the assembly for termination and splicing purposes. The hardness of the acrylate layer is such that at least the outermost fibres of the bundle are restricted from moving axially relative to the inner layer. The inner layer is then surrounded by a loose thin jacket formed from a mixture of high density polyethylene having a Shore hardness greater than or equal to 60 and a generally uniformly distributed slip agent, including a polyether modified poly (dimethylsiloxane) material such as polyether modified hydroxy functional poly (dimethylsiloxane) material. The mixture from which the outer layer is formed is compacted by means of heat and pressure. The outer layer may also contain a mineral filler, such as calcium carbonate and/or titanium dioxide, in order to improve the stability of the dimensions of the outer layer as the temperature changes.

The present invention relates to signal transmitting cables, and relatesparticularly, but not exclusively, to optical cables to be installed inducts by blowing.

EP0108590 discloses a process by which optical fibre cables areinstalled in ducts by means of fluid drag using the blowing method. Thisprocess now represents the most significant method by which opticalfibres are installed and there has been a great deal of development workaimed at optimizing both the installation performance and the signalcarrying performance of such cables.

This installation method, which distributes the installation forceevenly along the entire length of the cable, has enabled the developmentof cables which do not contain any reinforcement and which are verysmall and lightweight. This has brought new factors into play thataffect the installation performance. In particular, static electricitycan create a sufficiently strong attraction between the smalllightweight cable and the tube into which it is being installed tocreate very high levels of friction which might prevent satisfactoryinstallation being achieved.

There are a large number of other factors apart from friction and staticelectricity which affect the installation performance of a fibre opticcable. For example, the stiffness of the cable is important since theinstallation force is partly generated by pushing, the surface finish ofthe cable is important because it affects the viscous dragcharacteristics of the cable, and the pressure of the air and hence thevolume of air flow generated in the tube affect the installation forcegenerated from viscous drag.

GB2156837 discloses a method for improved insertion and withdrawal of anoptical fibre member by propelling the fibre member by means of fluiddrag through a pathway of a conduit which is obtained by the addition tothe conduit material, or the sheath material of the fibre member, of anadherence reducing substance such as an antistatic agent, slip agent, oranti-block agent, or a combination of these.

Attention has focussed on development of cables, as a result of whichthere has not been significant development of the tubes into which thecables are installed.

U.S. Pat. No. 4,740,053 describes an optical fibre cable comprising aninner sheath which may comprise a coating applied to the optical fibresor may be formed by an extrusion about the fibres. The inner sheathholds a plurality of optical fibres locked together in a unitary matrix.This has the benefit of providing stiffness, useful for pushing thecable into the duct in the initial phase of the process of blowing thecable into a duct. The outer sheath comprises cellular material of lowdensity and substantially greater cross sectional area than the innersheath. The material of the outer sheath may advantageously be chosen toaccept antistatic, antifriction agents and the like, and the outersheath is conveniently directly adhered to the inner sheath.

U.S. Pat. No. 4,952,021 discloses a similar arrangement to thatdisclosed in U.S. Pat. No. 4,740,053 above, but also discloses that theantistatic and antifriction agents can be incorporated in both the tubeand the outermost layer of the cable. In this case seven individualfibres are first coated with a solid layer of nylon to an outsidediameter of 1 mm and then a foamed low density polyethylene outer layeris applied to achieve a final outside diameter of 2 mm. Low densitypolyethylene is generally selected because it foams more easily thanhigh density polyethylene and creates a relatively soft outer layerwhich can be easily removed to expose the individual fibres.

This type of construction is thought to be beneficial primarily becausethe foam outer layer provides a large increase in diameter for a smallincrease in weight. A foam polyethylene is usually the polymer of choiceand typically the density of the material might be reduced from 0.93gms/cc to 0.5 gms/cc. This creates a large increase in diameter for arelatively small gain in weight and also produces a slightly roughsurface. Both these create an increase in viscous drag which is directlyrelated to the diameter of the object and also to the surface roughness.

This design suffers from some significant disadvantages, however. Thefriction characteristics of low density polyethylene are quite poordespite the addition of antifriction agents. Also, whilst the increasein diameter created by the foam outer layer increases fluid drag, italso serves to choke off the air flow down the tube as the cable isinstalled. This means that relatively large tubes need to be used toachieve satisfactory installation distances. The requirement isgenerally to use smaller tubes in order to optimise the use of alreadycongested networks. A further disadvantage is that manufacturing thefoam outer layer is problematic with inconsistencies in foam densityadversely affecting the optical properties and hence signal transmittingcapability of the fibres.

EP052171 0 discloses an alternative design for a fibre optic cable whichis much more compact and is designed to provide significantly improvedinstallation performance and in particular allow the use of smallertubes. This document discloses a fibre optic cable consisting of morethan one layer, where the outer surface or layer has been modified toobtain the benefit of increased fluid drag and reduced friction. A roughsurface has the benefit of increasing the effective outside diameterwithout increasing the weight to the same extent as a cable of the samediameter having a smooth external surface. Increasing the effectivediameter increases the fluid drag. In addition, rough surfacesintrinsically have higher fluid drag coefficients. Finally, roughsurfaces reduce the number of contact points between the cable and thetube and therefore reduce friction between the cable and the tube. Allof these factors improve installation characteristics and blowingdistances.

However, it is also well known that manufacturing such cables with roughsurfaces is problematic. In particular, the attachment of glassmicrospheres as a means of providing a rough surface is known to cause aweakening of the surface coating, which can create fibre break out,where the individual fibres break out of the coating, causingmicro-bending and create unacceptable signal losses. Another problem ofsuch cables is that the microspheres can become detached, creating apotential hazard during installation by blowing.

EP 646818 discloses a method for overcoming some of the disadvantages ofthis manufacturing technique by means of the application of threeseparate layers, making the process relatively complex, expensive andmore difficult to control.

Also, in the case of the prior art cables described in EP646818 andEP0521710, the different layers of the coating are bonded to each otheror at least in intimate contact with each other. In order to terminateor splice the cable it is necessary to break the individual fibres outof the coating layers. The individual fibres are quite delicate and thecoating layers are in intimate contact with the fibres. It is thereforeimportant that the coating layers are relatively soft and easy toremove. A disadvantage of such soft materials, however, is that theytend to have poor friction properties compared to harder materials andare more easily damaged, in particular by abrasion during installation.

U.S. Pat. No. 4,952,021 and U.S. Pat. No. 4,740,053 disclosearrangements in which all the layers of the coating are in intimatecontact with neighbouring layers. In the case of the cable of U.S. Pat.No. 4,740,053, the outer layer is conveniently directly adhered to theinner sheath. U.S. Pat. No. 4,740,053 states that the inner sheath isformed from a relatively high density material having a high modulus ofelasticity, and also a relatively hard and tough material. U.S. Pat. No.4,952,021 describes a cable in which seven fibres are first coated witha nylon layer, a relatively hard and tough material. It has been thepractise therefore for such cables to be provided with a rip cordpositioned adjacent to the fibres in the centre of the cable which couldbe pulled to cut open the tough inner layer so that access can be gainedfor terminating and splicing the individual fibres. A disadvantage ofthis approach, however, is that such rip cords are expensive andundesirably increase the size of the assembly.

Preferred embodiments of the present invention seek to overcome theabove disadvantages of the prior art.

According to an aspect of the present invention, there is provided acable assembly comprising a plurality of flexible signal transmittingmembers surrounded by a first layer such that axial movement of at leastthe outermost signal transmitting members relative to said first layeris restricted, and a continuous thermoplastic polymer second layerarranged outwardly of said first layer such that the hardness of thepolymer of the second layer is greater than or equal to a Shore Dhardness of 60.

The present invention is based on the surprising discovery thatlightweight fibre optic cables with excellent optical and blowingproperties can be manufactured by providing an outer layer of the cableformed from at least one polymer material, even in the case of a smoothouter layer, if the hardness of the outer layer is sufficient. Thepolymer may be conveniently modified to provide suitable antistatic andantifriction properties. This avoids the complex production problemsassociated with the production of a rough outer surface by theapplication of glass microspheres, foamed thermoplastics, and the like.This result is surprising firstly because the high fluid drag providedby a rough outer surface is generally regarded by persons skilled in theart as essential to providing good blowing performance. Secondly, whilstthe friction characteristics of the outer layer of the invention aregood relative to some polymers, they are inferior to the prior artarrangements, for example as described in EP 0521710. Very surprisinglythe blowing performance significantly exceeds the performance of theseprior art cables.

The hardness of the polymer of the second layer may be greater than orequal to a Shore hardness of 60 as measured by means of ISO R868.

The thickness of the second layer may be less than 400 microns around atleast 10% of the circumference of the cable assembly.

It has been found to be beneficial for the outer layer to be relativelythin and certainly thinner than the 0.5 mm of foamed low densitypolyethylene in the arrangement of U.S. Pat. No. 4,952,021. This has thebenefit that a harder material can be used without adversely affectingthe bending properties of the cable.

Harder materials provide more robust cables with better resistance toabrasion during installation and improved protection of the fragilesignal transmitting members. It is also the case that harder materialssuch as nylon or high density polyethylene have intrinsically betterfriction properties than other polymers comprising the outer layer ofother prior art cables such as low density polyethylene and acrylatepolymer. Thus it may not be necessary to modify the polymer withantifriction agents considerably reducing the cost of the material ofthe outer layer and the cost of the process.

The second layer preferably has a thickness of less than 200 micronsaround at least 10% of the circumference of the cable assembly.

The second layer preferably has a thickness of less than 125 micronsaround at least 10% of the circumference of the cable assembly.

In a preferred embodiment, the second layer is adapted to be removedfrom said first layer by sliding over said first layer.

It has also been found to be beneficial for the outer layer not to bebonded to the inner layer. Indeed it is preferable that a small gap beprovided between the two layers. This has the benefit that the outerhard polymeric material can be cut and removed from the inner layer bysliding it over the inner layer, providing easy access to the signaltransmitting members for termination or splicing. This avoids the needfor rip cords to longitudinally cut and remove the hard polymericsheath. A second advantage of the small gap between the layers is thatit provides an increase in diameter with no increase in weight, adesirable property for providing increased fluid drag and improvedinstallation performance.

According to another aspect of the present invention, there is provideda cable assembly comprising a plurality of flexible signal transmittingmembers surrounded by a first layer such that axial movement of at leastthe outermost signal transmitting members relative to said first layeris restricted, and a continuous thermoplastic polymer second layerarranged outwardly of said first layer and having a thickness of lessthan 400 microns around at least 10% of the circumference of the cableassembly.

The second layer preferably has a thickness of less than 200 micronsaround at least 10% of the circumference of the cable assembly.

The second layer preferably has a thickness of less than 125 micronsaround at least 10% of the circumference of the cable assembly.

According to a further aspect of the present invention, there isprovided a cable assembly comprising a plurality of flexible signaltransmitting members surrounded by a first layer such that axialmovement of at least the outermost signal transmitting members relativeto said first layer is restricted, and a continuous thermoplasticpolymer second layer arranged outwardly of said first layer and adaptedto be removed from said first layer by sliding over said first layer.

The inner periphery of said second layer may be longer than the outerperiphery of said first layer to enable removal of said second layerfrom the assembly.

The second layer may have a shore hardness greater than 60.

The second layer may comprise at least one polymer material.

At least one said polymer material may be a thermoplastic material.

At least one said polymer may be high-density polyethylene.

The flexible signal transmitting members may be embedded in said firstlayer.

Preferred embodiments of the present invention will now be described, byway of example only and not in any limitative sense, with reference tothe accompanying drawings, in which:

FIG. 1A is a schematic cross-sectional view of a fibre optic cable of afirst embodiment of the present invention;

FIG. 1B is a schematic cross-sectional view of a fibre optic cable of asecond embodiment of the present invention;

FIG. 1C is a schematic cross-sectional view of a fibre optic cable of athird embodiment of the present invention;

FIG. 1D is a schematic cross-sectional view of a fibre optic cable of afourth embodiment of the present invention;

FIG. 2 is a schematic representation of apparatus for manufacturing thecables of FIG. 1A to 1D;

FIG. 3 is a drawing of the test equipment used to measure thecoefficient of friction between cables and a tube suitable forinstallation of cables by blowing.

FIG. 4 a illustrates the speed of installation and the total installeddistance of the fibre optic cable of FIG. 1C into a duct, compared withthe performance of a prior art cable constructed with the surfacemodification described in EP 0521710 and EP 646818, also containing 8fibres;

FIG. 4 b illustrates the speed of installation and the total installeddistance of the fibre optic cable of FIG. 1 B into a duct, compared withthe performance of a prior art cable constructed with the surfacemodification described in EP 0521710 and EP 646818, also containing 12fibres; and

FIG. 5 illustrates optical attenuation characteristics of the cable ofFIG. 1B over a wide range of temperatures.

Referring to FIGS. 1A to 1D, a fibre optic cable 1 includes a core ofprimary coated optical fibres 2, which will be familiar to personsskilled in the art, embedded in an inner layer 3 of acrylate materialhaving sufficient tensile strength when cured to lock at least theoutermost fibres 2 in place and still allow the fibres to be easilybroken out of the assembly for termination and splicing purposes.Suitable materials for this application are DSM Cabelite 950-706 and DSMCabelite 3287-9-41. These materials are available from DSM Desotech BV.The hardness of the acrylate layer 3 is such that at least the outermostfibres 2 of the bundle are restricted from moving axially relative tothe inner layer 3.

The inner layer 3 is then surrounded by a loose thin jacket 4 formedfrom a mixture of high density polyethylene having a Shore hardnessgreater than or equal to 60 as measured by means of ISO R868 and agenerally uniformly distributed slip agent, including a polyethermodified poly (dimethylsiloxane) material such as polyether modifiedhydroxy functional poly (dimethylsiloxane) material. The mixture fromwhich the outer layer 4 is formed is compacted by means of heat andpressure. The outer layer 4 may also contain a mineral filler, such ascalcium carbonate and/or titanium dioxide, in order to improve thestability of the dimensions of the outer layer 4 as the temperaturechanges.

In order to manufacture the cables 1 of FIGS. 1A to 1D, the primarycoated optical fibres 2 are supplied from a bank of payoff reels (notshown), the number of reels being equal to the number of fibres 2 to beincluded in the cable 1. The fibres 2 are unwound with a generallyconstant traction force. The fibres 2 are then bundled together into abundle of suitable shape, and are passed through a resin applicationstation, where an acrylate resin forming the inner layer 3 is applied tothe bundle of fibres 2, the acrylate resin being a UV-curing resin. Thecoated assembly of fibres 2 is then pulled through a series of curingovens which cure the inner layer 3 to the desired dimensions. The aboveprocess can be carried out, for example, using a modified fibre ribbonline provided by Nextrom, Vantaa, Helsinki, Finland.

Referring now to FIG. 2, the external coating 4, formed from a mixtureof polymer and friction reducing material which has previously beencompounded by means of heat and pressure, is applied to the inner layer3 of the coated optical fibre bundle described above by pulling thecoated fibre bundle through a thermoplastic extrusion line as shown inFIG. 2. Such a line is available from Nextrom Technologies, Nextrom S A,Route du Bois, 37 PO Box 259, CH-1024 Ecublens-Lausanne, Switzerland.The thermoplastic extrusion line 10 has a payoff stand 11 which allowsthe coated fibre bundle to be paid off a reel 12 at a generally steadyrate. A tensioning device 13 ensures that the coated bundle is tautbefore entering an extrusion crosshead 14, which applies the mixture ofhigh-density polyethylene incorporating the suitable silicon slip agentto the coated bundle at a temperature between 190 degrees C and 230degrees C.

The polyethylene coated cable is then pulled through a vacuum tank 15which applies a vacuum to the outer coating 4 by surrounding it withwater, the vacuum being between 100 mbar and 50 mbar, and also cools thefibre unit as it leaves the extrusion crosshead 14. Additional coolingis provided by pulling the cable through a water trough 16, the waterbeing at a temperature of approximately 20 degrees C. A caterpillar unit17 pulls the fibre unit through the entire thermoplastic extrusion line10, the cable 1 then being coiled into a pan 18 by means of a coiler 19.It will be appreciated by persons skilled in the art that the twoprocesses described above could be arranged in a single manufacturingline and the process completed in a single stage.

Referring to FIG. 3, this shows an apparatus for measuring the frictioncharacteristics of the cables. Two cables, the first embodying thepresent invention and the second a commercially available cable with thesurface modification described in EP 0521710 and EP 646818 were testedto measure their coefficient of friction relative to a tube manufacturedcommercially for use in blown cable applications.

The test method comprises attaching a weight of 10 grammes to one end ofthe cable and threading the other end through tube 101, around pulley102, through tube 103 and then through a length of tube 104. The tube104 is a commercially available tube with outside diameter 5 mm andinternal diameter 3.5 mm manufactured for receiving installation ofcables by blowing. The tube 104 is wrapped around a wheel 105 to providea total of 450 degrees of wrapping. After the cable has been threadedthrough the tube 104 it is then inserted into a haul off 106, whichpulls the cable at a constant speed of 10 metres per minute. The tube104 is clamped at both ends by clamps 107, and as the cable is pulledthrough the tube 104, the friction of the cable on the tube imposes aturning moment on the wheel 105 and rotates a lever 108 which imposes aload on a mass balance 109.

The load on the mass balance 109 was measured for both the invention andthe prior art and the coefficient of friction calculated using theformula:

Coefficient of friction is given by:$\mu = {\frac{1}{\theta}{\ln\left\lbrack {\frac{FL}{Tr} + 1} \right\rbrack}}$Where

-   -   θ total wrap angle of tube (rads)    -   F force recorded at mass balance (N)    -   L Moment arm length of force F (m)    -   T Weight lifted by fibre (N)    -   r Bend radius of primary tube (m)

The cable of the invention had a coefficient of friction of 0.27 whilstthe cable of the prior art had a coefficient of friction of 0.21. Thefriction characteristics of the invention are therefore inferiorcompared to those of the prior art.

Referring now to FIGS. 4 a and 4 b, the blowing performance of thecable, manufactured according to the above process is assessed bymeasuring the speed of installation and the total distance installed ofthe fibre unit into a suitable duct. The comparison involves an industrystandard test in which 500 metres of a commercially available tube withoutside diameter 5 mm and internal diameter 3.5 mm manufactured forreceiving installation of cables by blowing, is wound onto a drum withbarrel diameter of 500 mm.

In the case of FIG. 4 a, two fibre optic cables, the first being thecable of FIG. 1C (curve A) and the second a commercially available cablewith the surface modification described in EP 0521710 and EP 646818(curve B) are compared. Each of the cables contained 8 fibres arrangedin their respective coatings. The cables of prior art and the inventionwere blown into the tube using industry standard blowing equipment,compressed air at 10 bar pressure and techniques identical for bothcables.

In FIG. 4 a, the blowing performance of the two cables is compared. Itcan be seen that the prior art product started to slow down after only250 metres had been installed. At 430 metres the installation speed haddeclined to only 10 m/min. The cable of the invention, on the otherhand, completed the test route at a constant speed of 24 m/min. In FIG.4 b the comparison is repeated except that this time the cables eachcontained 12 fibres, i.e. the cable of the invention is the cable ofFIG. 1B. In this case the prior art cable (curve D) installed just 24metres before stopping completely whilst the cable of the invention(curve C) completed a distance of 375 metres before stopping.

The blowing performance of FIGS. 4 a and 4 b represents a substantialand unexpected improvement compared to the prior art, particularly so inview of the fact that the cable of the invention has inferior frictionproperties and has a surface which had not been physically modified inany way to enhance fluid drag.

Referring now to FIG. 5, the signal loss over a wide temperature rangeassociated with cables manufactured according to the above process isshown. The different curves show signal attenuation in the individualfibres 2 of the cable of FIG. 1B. It can be seen that the cable 1 canwithstand exposure to a wide temperature range. This is a surprisingresult. Prior art cables as described in EP0157610 incorporatingpolyethylene outer layers display poor optical performance belowapproximately B20 □C. This is usually attributed to a change of phase inpolyethylene at around this temperature and for this reason polyethyleneis not normally selected for the tight jacketing of fibre opticelements.

It will be appreciated by persons skilled in the art that the aboveembodiments have been described by way of example only, and not in anylimitative sense, and that various alterations and modifications arepossible with departure from the scope of the invention as defined bythe appended claims. For example, as an alternative to, or in additionto, the friction reducing materials described in the above embodiments,erucamide and/or oleamide materials may be used as slip agents.Furthermore, although the cable assembly of the present inventioncomprises an inner and an outer layer, however it will be obvious tothose skilled in the art that it might be constructed from more than twolayers.

1. A cable assembly, comprising a plurality of flexible signaltransmitting members surrounded by a first layer, wherein axial movementof at least the outermost of said signal transmitting members relativeto said first layer is restricted, and further comprising a continuousthermoplastic polymer second layer arranged outwardly of said firstlayer, wherein the hardness of the polymer of said second layer isgreater than or equal to a Shore D hardness of
 60. 2. The cable assemblyof claim 1, wherein the hardness of the polymer of said second layer isgreater than or equal to a Shore hardness of 60 as measured by means ofISO R868.
 3. The cable assembly of claim 1, wherein the thickness ofsaid second layer is less than 400 microns around at least 10% of thecircumference of said cable assembly.
 4. The cable assembly of claim 3,wherein said second layer has a thickness of less than 200 micronsaround at least 10% of the circumference of said cable assembly.
 5. Thecable assembly of claim 4, wherein said second layer has a thickness ofless than 125 microns around at least 10% of the circumference of saidcable assembly.
 6. The cable assembly of claim 1, wherein said secondlayer is adapted to be removed from said first layer by sliding oversaid first layer.
 7. The cable assembly of claim 6, wherein the innerperiphery of said second layer is longer than the outer periphery ofsaid first layer.
 8. The cable assembly of claim 1, wherein said innerlayer comprises at least one acrylate material.
 9. A cable assembly,comprising a plurality of flexible signal transmitting memberssurrounded by a first layer, wherein axial movement of at least theoutermost of said signal transmitting members relative to said firstlayer is restricted, and further comprising a continuous thermoplasticpolymer second layer arranged outwardly of said first layer, said secondlayer having a thickness of less than 400 microns around at least 10% ofthe circumference of said cable assembly.
 10. The cable assembly ofclaim 9, wherein said second layer has a thickness of less than 200microns around at least 10% of the circumference of said cable assembly.11. The cable assembly of claim 10, wherein said second layer has athickness of less than 125 microns around at least 10% of thecircumference of said cable assembly.
 12. The cable assembly of claim 9,wherein the hardness of the polymer of said second layer is greater thanor equal to a Shore D hardness of
 60. 13. The cable assembly of claim 9,wherein the hardness of the polymer of said second layer is greater thanor equal to a Shore hardness of 60 as measured by means of ISO R868. 14.The cable assembly of claim 9, wherein said second layer is adapted tobe removed from said first layer by sliding over said first layer. 15.The cable assembly of claim 14, wherein the inner periphery of saidsecond layer is longer than the outer periphery of said first layer. 16.A cable assembly, comprising a plurality of flexible signal transmittingmembers surrounded by a first layer such that axial movement of at leastthe outermost signal transmitting members relative to said first layeris restricted, and further comprising a continuous thermoplastic polymersecond layer arranged outwardly of said first layer and adapted to beremoved from said first layer by sliding over said first layer.
 17. Thecable assembly of claim 16, wherein the inner periphery of said secondlayer is longer than the outer periphery of said first layer to enableremoval of said second layer from said assembly.
 18. The cable assemblyof claim 16, wherein said second layer has a Shore hardness greater than60.
 19. The cable assembly of claim 18, wherein the hardness of thepolymer of said second layer is greater than or equal to a Shorehardness of 60 as measured by means of ISO R868.
 20. The cable assemblyof claim 16, wherein the thickness of said second layer is less than 400microns around at least 10% of the circumference of said cable assembly.21. The cable assembly of claim 20, wherein said second layer has athickness of less than 200 microns around at least 10% of thecircumference of said cable assembly.
 22. The cable assembly of claim21, wherein said second layer has a thickness of less than 125 micronsaround at least 10% of the circumference of said cable assembly.
 23. Theassembly of claim 16, wherein said second layer comprises at least onepolymer material.
 24. The assembly of claim 23, wherein said at leastone polymer material comprises a thermoplastic material.
 25. Theassembly of claim 23, wherein said at least one polymer materialcomprises high-density polyethylene.
 26. The assembly of claim 16,wherein said signal transmitting members are embedded in said firstlayer.